The present invention relates to an intradiscal trial implant device. More particularly, it relates to a trial implant and a trial implant kit for use in evaluating an intradiscal space, including an anulus and nucleus cavity, and for assisting a surgeon in the selection of an appropriately sized prosthetic spinal disc nucleus.
The vertebral spine is the axis of the skeleton upon which all of the body parts "hang". In humans, the normal spine has seven cervical, twelve thoracic and five lumbar segments. The lumbar segments sit upon a sacrum, which then attaches to a pelvis, in turn supported by hip and leg bones. The bony vertebral bodies of the spine are separated by intervertebral discs, which act as joints, but allow known degrees of flexion, extension, lateral bending and axial rotation.
The typical vertebra has a thick interior bone mass called the vertebral body, with a neural (vertebral) arch that arises from a posterior surface of the vertebral body. Each neural arch combines with the posterior surface of the vertebral body and encloses a vertebral foramen. The vertebral foramina of adjacent vertebrae are aligned to form a vertebral canal, through which the spinal sac, cord and nerve rootlets pass. The portion of the neural arch that extends posteriorly and acts to protect a posterior side of the spinal cord is known as a lamina. Projecting from the posterior region of the neural arch is a spinous process. The central portions of adjacent vertebrae are supported by the intervertebral disc.
The intervertebral disc primarily serves as a mechanical cushion between the vertebral bones, permitting controlled motions within vertebral segments of the axial skeleton. The normal disc is a unique, mixed structure, comprised of three component tissues. The nucleus pulposus ("nucleus"), the anulus fibrosus ("anulus"), and two opposing vertebral end plates. The two vertebral end plates are each composed of thin cartilage overlying a thin layer of hard, cortical bone which attaches to the spongy, richly vascular, cancellous bone of the vertebral body. The end plates thus serve to attach adjacent vertebrae to the disc. In other words, a transitional zone is created by the end plates between the malleable disc and the bony vertebrae.
The anulus of the disc is a tough, outer fibrous ring that binds together adjacent vertebrae. This fibrous portion, which is much like a laminated automobile tire, is generally about 10 to 15 millimeters in height and about 15 to 20 millimeters in thickness. The fibers of the anulus consist of 15 to 20 overlapping multiple plies, and are inserted into the superior and inferior vertebral bodies at roughly a 30-degree angle in both directions. This configuration particularly resists torsion, as about half of the angulated fibers will tighten when the vertebrae rotate in either direction, relative to each other. The laminated plies are less firmly attached to each other.
Immersed within the anulus, positioned much like the liquid core of a golf ball, is the nucleus. The anulus and opposing end plates maintain a relative position of the nucleus in what can be defined as a nucleus cavity. The healthy nucleus is largely a gel-like substance having a high water content, and similar to air in a tire, serves to keep the anulus tight yet flexible. The nucleus-gel moves slightly within the anulus when force is exerted on the adjacent vertebrae with bending, lifting, etc.
The nucleus and the inner portion of the anulus have no direct blood supply. In fact, the principal nutritional source for the central disc arises from circulation within the opposing vertebral bodies. Microscopic, villous-like fingerlings of the nuclear and anular tissue penetrate the vertebral end plates and allow fluids to pass from the blood across the cell membrane of the fingerlings and then inward to the nuclear tissue. These fluids are primarily body water and the smallest molecular weight nutrients and electrolytes.
The natural physiology of the nucleus promotes these fluids being brought into, and released from, the nucleus by cyclic loading. When fluid is forced out of the nucleus, it passes again through the end plates and then back into the richly vascular vertebral bodies. The cyclic loading amounts to daily variations in applied pressure on the vertebral column (e.g., body weight and muscle pull) causing the nucleus to expel fluids, followed by periods of relaxation and rest, resulting in fluid absorption or swelling by the nucleus. Thus, the nucleus changes volume under loaded and non-loaded conditions. Further, the resulting tightening and loosening effect on the anulus stimulates the normal anulus collagen fibers to remain healthy or to regenerate when torn, a process found in all normal ligaments related to body joints. Notably, the ability of the nucleus to release and imbibe fluids allows the spine to alter its height and flexibility through periods of loading or relaxation. Normal load cycling is thus an effective nucleus and inner anulus tissue fluid pump, not only bringing in fresh nutrients, but perhaps more importantly, removing the accumulated, potentially autotoxic by-products of metabolism.
The spinal disc may be displaced or damaged due to trauma or a disease process. A disc herniation occurs when the anulus fibers are weakened or torn and the inner tissue of the nucleus becomes permanently bulged, distended, or extruded out of its normal, internal anular confines. The mass of a herniated or "slipped" nucleus can compress a spinal nerve, resulting in leg pain, loss of muscle control, or even paralysis. Alternatively, with discal degeneration, the nucleus loses its water binding ability and deflates, as though the air had been let out of a tire. Subsequently, the height of the nucleus decreases, causing the anulus to buckle in areas where the laminated plies are loosely bonded. As these overlapping laminated plies of the anulus begin to buckle and separate, either circumferential or radial anular tears may occur, which may contribute to persistent and disabling back pain. Adjacent, ancillary spinal facet joints will also be forced into an overriding position, which may create additional back pain.
Whenever the nucleus tissue is herniated or removed by surgery, the disc space will narrow and may lose much of its normal stability. In many cases, to alleviate pain from degenerated or herniated discs, the nucleus is removed and the two adjacent vertebrae surgically fused together. While this treatment alleviates the pain, all discal motion is lost in the fused segment. Ultimately, this procedure places greater stress on the discs adjacent the fused segment as they compensate for the lack of motion, perhaps leading to premature degeneration of those adjacent discs. A more desirable solution entails replacing in part or as a whole the damaged nucleus with a suitable prosthesis having the ability to complement the normal height and motion of the disc while stimulating the natural disc physiology.
The first prostheses embodied a wide variety of ideas, such as ball bearings, springs, metal spikes and other perceived aids. These prosthetic discs were designed to replace the entire intervertebral disc space (as opposed to only the nucleus), and were large and rigid. Beyond the questionable efficacy of these devices was the inherent difficulties encountered during implantation. Due to their size and inflexibility, these devices required an anterior implantation approach as the barriers presented by the lamina and, more importantly, the spinal cord and nerve rootlets during posterior implantation could not be avoided. Recently, smaller and more flexible prosthetic nucleus devices have been developed. With the reduction in prosthesis size, the ability to work around the spinal cord and nerve rootlets with a posterior implantation is now possible.
For example, Ray et al., U.S. Pat. No. 5,647,295 discloses a hydrogel-based prosthetic nucleus that is implanted into the intradiscal space in a dehydrated state. The Ray et al. prosthesis includes a jacket sized to constrain expansion of the hydrogel core. More particularly, following implant, the constraining jacket directs the hydrogel to expand primarily in height, thereby separating adjacent vertebrae. The prosthetic spinal disc nucleus of Ray et al. is sized such that in a final hydrated form, the prosthesis has a volume much less than a volume of the nucleus cavity. In this way, two prostheses can be orientated in a side-by-side fashion within the nucleus cavity. With this dual-prosthesis approach, only a small incision in the anulus is required for implantation, thereby limiting damage to the anulus.
Variations to the Ray et al. prosthesis have been envisioned, including pre-shaping the hydrogel core to more closely correspond with the inherent shape of a portion of a particular nucleus cavity. More particularly, the hydrogel core may be pre-formed to assume a wedge shape to accommodate height variations at the anterior or posterior side of the intradiscal space.
While the device of Ray et al., along with the above-described variations and other similar products, are clearly beneficial, selection of a properly sized prosthesis may be difficult. In this regard, while the individual intradiscal spaces making up the human spine are, relatively speaking, similar, distinctions in terms of shape and size exist. For example, a central area of the L4-L5 intradiscal space has an increased height at a central portion in comparison with the posterior and anterior sides. Conversely, the L5-S1 intradiscal space typically has an essentially uniform increase in height from the posterior side to the anterior side. Even further, the shape and size characteristics of a particular disc space may vary greatly from person to person. Finally, unforeseen impediments within the intradiscal space itself may limit the area available to receive the prosthesis. For example, Ray et al. describes preferably removing the entire nucleus prior to implanting the prosthetic spinal disc nuclei. While every effort is made to accomplish this result, invariably some nucleus tissue remains within the nucleus cavity. This excess tissue may reduce the available area of the nucleus cavity, thereby limiting proper implantation of a certain sized prosthesis.
While the Ray et al. prostheses, and variations thereof, do not rely upon the anulus for constraining the hydrogel core, certain dimensional concerns remain. It is generally desirable that each of the two implanted prostheses corresponds as closely as possible to the region of the intradiscal space receiving the implant. For example, with Ray et al., a first one of the two prostheses may be implanted at the anterior side of the intradiscal space, extending transversely within the nucleus cavity. As a point of reference, the second prosthesis is implanted at the posterior side of the intradiscal space, again extending in a transverse fashion. With this orientation, the first prosthesis must be sized to have a length approximating a transverse diameter of the nucleus cavity at the anterior side thereof. Additionally, the prosthesis must be sized to satisfy the above-described height variations of a nucleus cavity.
In light of the above, it may be difficult for a surgeon to accurately select an appropriately sized prosthetic spinal disc nucleus. Due to the enclosed nature of the nucleus cavity, it is virtually impossible for the surgeon to accurately evaluate the size and shape of a particular nucleus cavity. In this regard, X-rays reveal little, if any, of the details of a discal region. As a result, the surgeon is normally forced to estimate the nucleus cavity based upon the patient's height, weight, the particular intradiscal space in question, etc. While this method of estimation is normally sufficient, occasionally an incorrectly sized prosthesis is selected, leading to possible problems. If, for example, too large a prosthesis is chosen, the surgeon will be unable to optimally position the device within the nucleus cavity. In fact, the surgeon may find it impossible to insert the prosthesis into the disc space or to achieve proper orientation. Conversely, where the selected prosthesis is too small, sufficient support may not be provided, potentially resulting in disc failure. Unfortunately, in either case, the surgeon will not be aware of the sizing problem until after he or she has attempted to implant the prosthetic spinal disc nucleus. As a result, the prosthetic spinal disc nucleus will have been in direct contact with blood and other bodily fluids. Where the prosthetic spinal disc nucleus includes a woven outer jacket, or a cover composed of a similar material, it is virtually impossible to resterilize the prosthesis. Thus, the prosthetic spinal disc nucleus, normally an expensive device, must be discarded.
The prosthetic spinal disc nucleus has been shown to be a highly useful tool for correcting degenerated intervertebral discs. However, the inability to accurately evaluate a disc space prior to implant may lead to complications. Therefore, a need exists for a device or kit used to evaluate an available internal area of an intradiscal space and assist in selecting a properly sized prosthetic spinal disc nucleus.